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I thought I would quickly pen this article having recently run into some awkward problems while tackling visualisations of some extremely confined interior spaces. I am sharing this in case it may help or offer some pointers to anyone else tasked with a similar problem. I am using my reconstructions of Orford Castle in Suffolk, produced for English Heritage, as an example here.

When setting up my 3D models in the Blender 3D software, the usual and most obvious way to fit most of the scene into the final image is to set a very wide angle camera lens. But the drawback with wide angle lens settings is that they tend to create a lot of distortion in the image which just doesn't look visually pleasing. So, I prefer to use longer focal lengths of between 40mm and 70mm but that means of course, that we won't get to see all of the room in the image. How did I tackle this problem, then?

Well, I cheated.

In Blender 3D, you can bend the rules somewhat. There are some nifty camera settings that emulate often prohibitively-expensive tilt-shift lenses which are used a lot by architectural photographers. Using a combination of camera rotation and adjusting the camera's X and Y shift values, it's possible to correct some of the distortion that comes with the wider angle lens settings. You can also play with the camera's sensor size and choose a setting that emulates a real full-frame camera - and even beyond that! But sometimes even this doesn't quite provide enough flexibility. So this is when it's time to roll out the big hacks.

When visualising Orford Castle's prison, I had faced the following challenges:

I needed to show as much of the interior space as possible,

The room is VERY dark - there is only one tiny natural light source,

I needed to make the image very tall so that we can see how the prison was accessed via a rope-ladder and trap-door,

I have to minimise the amount of lens distortion in the image to create a pleasing image,

The scene needed to be accurately lit.

Positioning the camera inside the room with a wide angle of view resulted in this extremely distorted and completely unsatisfactory image.

I decided quite early on with this scene that choosing a wide angle lens wouldn't work here. What I needed was a way to position my camera at a distance just far enough away from my model that I could use a focal length of 60mm-70mm to avoid distortion and fit in as much of the prison's interior space as I could.

To do this I positioned my camera well outside of the model and set its image dimensions to 2500 x 5000 px which will produce for me a nice, detailed render of the full prison space. But now I have a wall with a backface obscuring the view of the prison's interior. It might seem like a straight-forward task of just deleting or hiding the wall in the scene, but the problem is that this hides the wall from both the camera AND the scene. The environment lighting that I have set up to illuminate the room (through a small window) penetrates the room through the void left by the missing wall, which is no good at all.

The surprisingly simple workaround here is to employ Blender's Light Path material node which controls what elements in a scene the camera will see in the render. There are some good tutorials out there on the web which explain this very useful node in more detail, but for my example I am using the Is Camera Ray output of the Light Path node as the factor for mixing two of Blender's material shaders - the Principled and Transparent shaders. This is making the obscuring wall transparent or invisible to the camera, but still visible to the rest of the scene so that lighting can be calculated accurately.

By positioning my camera some distance outside the room and using Blender's Light Path node to hide the backfacing wall from the camera, I could use a much longer focal length resulting in far less image distortion.

The final result of the render. The Prison, Orford Castle. Accurately-lit and no ugly lens distortion!

I used this same technique on Orford Castle's kitchens (image at top of page) - although in this example I did use a slightly wider camera focal length, with some distortion in the lower part of the image, it still gave some reasonably pleasing results.